Apparatus for audibly communicating speech using the radio frequency hearing effect

ABSTRACT

A modulation process with a fully suppressed carrier and input preprocessor filtering to produce an encoded output; for amplitude modulation (AM) and audio speech preprocessor filtering, intelligible subjective sound is produced when the encoded signal is demodulated using the RF Hearing Effect. Suitable forms of carrier suppressed modulation include single sideband (SSB) and carrier suppressed amplitude modulation (CSAM), with both sidebands present.

STATEMENT OF GOVERNMENT INTEREST

[0001] The invention described herein may be manufactured and used by orfor the Government for governmental purposes without the payment of anyroyalty thereon.

BACKGROUND OF THE INVENTION

[0002] This invention relates to the modulating of signals on carriers,which are transmitted and the signals intelligibly recovered, and moreparticularly, to the modulation of speech on a carrier and theintelligible recover of the speech by means of the Radio FrequencyHearing Effect.

[0003] The Radio Frequency (“RF”) Hearing Effect was first noticedduring World War II as a subjective “click” produced by a pulsed radarsignal when the transmitted power is above a “threshold” level. Belowthe threshold level, the click cannot be heard.

[0004] The discovery of the Radio Frequency Hearing Effect suggestedthat a pulsed RF carrier could be encoded with an amplitude modulated(“AM”) envelope. In one approach to pulsed carrier modulation, it wasassumed that the “click” of the pulsed carrier was similar to a datasample and could be used to synthesize both simple and complex tonessuch as speech. Although pulsed carrier modulation can induce asubjective sensation for simple tones, it severely distorts the complexwaveforms of speech, as has been confirmed experimentally.

[0005] The presence of this kind of distortion has prevented the clickprocess for the encoding of intelligible speech. An example is providedby AM sampled data modulation

[0006] Upon demodulation the perceived speech signal has some of theenvelope characteristics of an audio signal. Consequently a message canbe recognized as speech when a listener is pre-advised that speech hasbeen sent. However, if the listener does not know the content of themessage, the audio signal is unintelligible.

[0007] The attempt to use the click process to encode speech has beenbased on the assumption that if simple tones can be encoded, speech canbe encoded as well, but this is not so.

[0008] A simple tone can contain several distortions and still beperceived as a tone whereas the same degree of distortion applied tospeech renders it unintelligible.

SUMMARY OF THE INVENTION

[0009] In accomplishing the foregoing and related object the inventionuses a modulation process with a fully suppressed carrier andpre-processor filtering of the input to produce an encoded output. Whereamplitude modulation (AM) is employed and the preprocessor filtering isof audio speech input, intelligible subjective sound is produced whenthe encoded signal is demodulated by means of the RF Hearing Effect.Suitable forms of carrier suppressed modulation include single sideband(SSB) and carrier suppressed amplitude modulation (CSAM), with bothsidebands present.

[0010] The invention further provides for analysis of the RE hearingphenomena based on an RF to acoustic transducer model. Analysis of themodel suggests a new modulation process which permits the RF HearingEffect to be used following the transmission of encoded speech.

[0011] In accordance with one aspect of the invention the preprocessingof an input speech signal takes place with a filter that de-emphasizesthe high frequency content of the input speech signal. The de-emphasiscan provide a signal reduction of about 40 dB (decibels) per decade.Further processing of the speech signal then takes place by adding abias level and taking a root of the predistorted waveform. The resultantsignal is used to modulated an RF carrier in the AM fully suppressedcarrier mode, with single or double sidebands.

[0012] The modulated RF signal is demodulated by an RF to acousticdemodulator that produces an intelligible acoustic replication of theoriginal input speech.

[0013] The RF Hearing Effect is explained and analyzed as a thermal toacoustic demodulating process. Energy absorption in a medium, such asthe head, causes mechanical expansion and contraction, and thus anacoustic signal.

[0014] When the expansion and contraction take place in the head of ananimal, the acoustic signal is passed by conduction to the inner earwhere it is further processed as if it were an acoustic signal from theouter ear.

[0015] The RF to Acoustic Demodulator thus has characteristics whichpermit the conversion of the RF energy input to an acoustic output.

[0016] Accordingly, it is an object of the invention to provide a noveltechnique for the intelligible encoding of signals. A related object isto provide for the intelligible encoding of speech.

[0017] Another object of the invention is to make use of the RadioFrequency (“RF”) Hearing Effect in the intelligible demodulation ofencoded signals, including speech.

[0018] Still another object of the invention is to suitably encode apulsed RF carrier with an amplitude modulated (“AM”) envelope such thatthe modulation will be intelligibly demodulated by means of the RFHearing Effect. A related object is to permit a message to be identifiedand understood as speech when a listener does not know beforehand thatthe message is speech.

[0019] Other aspects of the invention will be come apparent afterconsidering several illustrative embodiments, taken in conjunction withthe drawings.

DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a block diagram model of RF to Acoustic DemodulationProcess making use of the Radio Frequency (“RF”) Hearing Effect;

[0021]FIG. 2 is a spherical demodulator and radiator having a specificacoustic impedance for demodulation using the RF Hearing Effect;

[0022]FIG. 3 is a diagram illustrating the overall process andconstituents of the invention; and

[0023]FIG. 4 is an illustrative circuit and wiring diagram for thecomponents of FIG. 3.

DETAINED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024] With reference to the drawings, FIG. 1 illustrates the RF toacoustic demodulation process of the invention. Ordinarily and acousticsignal A reaches the outer ear E of the head H and traverses first tothe inner ear I and then to the acoustic receptors of the brain B. Amodulated RF signal, however, enters a demodulator D, which isillustratively provided by the mass M of the brain, and is approximated,as shown in FIG. 2, by a sphere S of radius r in the head H. The radiusr of the sphere S is about 7 cm to make the sphere S equivalent to aboutthe volume of the brain B. It will be appreciated that where thedemodulator D, which can be an external component, is not employed withthe acoustic receptors of the brain B, it can have other forms.

[0025] The sphere S, or its equivalent ellipsoid or similar solid,absorbs RF power which causes an increase in temperature that in turncauses an expansion and contraction which results in an acoustic wave.As a first approximation, it is assumed that the RF power is absorbeduniformly in the brain. Where the demodulator D is external to the brainB, the medium and/or RF carrier frequency can be selected to assuresufficiently uniform absorption.

[0026] For the modulated RF signal of FIG. 1, the power absorbed in thesphere S is proportional to the power waveform of the modulated RFsignal. The absorption rate is characterized quantitatively in terms ofthe SAR (Specific Absorption Rate) in the units of absorbed watts perkilogram per incident watt per square centimeter.

[0027] The temperature of the sphere S is taken as following theintegrated heat input from the power waveform, i.e. the process isapproximated as being adiabatic, at least for short term intervals onthe order of a few minutes.

[0028] The radial expansion of the sphere follows temperature and isconverted to sound pressure, p(t), determined by the radial velocity(U_(r)) multiplied by the real part of the specific acoustic impedance(Z_(s)) of the sphere, as indicated in equation (1), below.

Z _(s)=ρ_(o) c(jkr)/(1+jkr)=ρ_(o) c jf/f _(c)/(1+jf/f _(c))  (1)

[0029] Where:

[0030] ρ_(o)=density, 1000 kg/m³ for water

[0031] c=speed of sound, 1560 m/s, in water @ 37° C.

[0032] k=wave number, 2π/wavelength

[0033] r=sphere radius, in meters (m)

[0034] f=audio frequency

[0035] f_(c)=lower cutoff break frequency,=c/(2πr)

[0036] j=the 90 degree phase-shift operator

[0037] The specific acoustic impedance for a sphere of 7 cm radius, onthe order of the size of the brain, has a lower cut-off break frequencyof about 3,547 Hertz (Hz) for the parameters given for equation (1). Theessential frequency range of speech is about 300 to 3000 Hz, i.e., belowthe cut-off frequency. It is therefore the Real part (R_(e)) of Z_(s)times the radial particle velocity (U_(r)) which determines the soundpressure, p(t). The real part of Z_(s) is given by equation (1a), below:

R _(e)(Z _(s))=ρ_(o) c(f/f _(c))²/(1+(f/f _(c))²)  (1a)

[0038] In the speech spectrum, which is below the brain cut-offfrequency, the sphere S is an acoustic filter which “rolls off”, i.e.decreases in amplitude at −40 dB per decade with decreasing frequency.In addition to any other demodulation processes to be analyzed below,the filter characteristics of the sphere will modify the acoustic signalwith a 40 dB per decade slope in favor of the high frequencies.

Results for an AM Modulated Single Tone

[0039] An RF carrier with amplitude A_(c) at frequency ω_(c) is AMmodulated 100 percent with a single tone audio signal at frequencyω_(a). The voltage (time) equation of this modulated signal is given byequation (2), below:

V(t)=A _(c) sin (ω_(c) t)(1+sin (ω_(a) t))  (2)

[0040] The power signal is V(t)² as given by equation (3), below:

P(t)=A _(c) ²[¾+sin(ω_(a) t)−¼ cos(2ω_(a) t)−{fraction (3/4)} cos(2ω_(c)t)−cos(2ω_(c) t) sin(ω_(a) t)+{fraction (1/4)} cos(2ω_(c) t) cos(2ω_(a)t)]  (3)

[0041] To find the energy absorbed in the sphere, the time integral ofequation (3) is taken time absorption coefficient, K. The result isdivided by the specific heat, SH to obtain the temperature of the sphereand then multiplied by the volume expansion coefficient, Mv to obtainthe change in volume. The change in volume is related to the change inradius by equation (4), below:

dV/V=3dr/r  (4)

[0042] To obtain the amplitude of the radius change, there ismultiplicauon by the radius and division by three. The rms radialsurface velocity, U_(r) is determined by multiplying the time derivativeby r and dividing by 2^(½). The result, U_(r), is proportional to thepower function, P(t) in equation (5), below.

U _(r)=0.3535 P(t)rKM _(v)/(3SH)  (5)

[0043] The acoustic pressure, p(t), is given in equation (6), below, asthe result of multiplying equation (5) by the Real part of the specificacoustic impedance, R_(e) (1).

p(t)=R _(e) {Z _(s) U _(r) }=R _(e)(Z _(s))U _(r)  (6)

[0044] The SPL (Sound Pressure Level), in acoustic dB, is approximatedas 20 log[p(t)/2E−5]. The standard acoustic reference level of 2E−5Newtons per square meter is based on a signal in air; however, the headhas a water-like consistency. Therefore, the subjective level inacoustic dB is only approximate, but sufficient for first orderaccuracy.

[0045] In a single tone case the incident RF power, P(t), from equation(3) has two terms as shown in equation (7), below, which are in thehearing range.

sin(ω_(a) t)−¼ cos(2ω_(a) t)  (7)

[0046] This is converted to the acoustic pressure wave, p(t), bymultiplying by the specific acoustic impedance calculated at the twofrequencies. Therefore, the resulting pressure wave as indicated inequation (8), below, becomes

p(t)=C[Z _(s)(ω_(a))sin(ω_(a) t)−¼Z _(s)(2ω_(a))cos(2ω_(a) t)]  (8)

[0047] The result is an audio frequency and a second harmonic at about ¼amplitude. Thus using an RF carrier, AM modulated by a single tone, thepressure wave audio signal will consist of the audio tone and a secondharmonic at about −6 dB, if the specific acoustic impedances at the twofrequencies are the same. However, from equation (1) the break frequencyof a model 7 cm sphere is 3.547 Hz. Most of the speech spectrum is belowthis frequency therefore the specific acoustic impedance is reactive andthe real component is given by equation (8a), below:

R _(e) {Z _(s)(f)}=ρ_(o) c(f/f _(c))²/(1+(f/f _(c)))  (8a)

[0048] Below the cutoff frequency the real part of the impedance variesas the square of the frequency or gives a boost of 40 dB per decade.Therefore, if the input modulation signal is 1 kHz, the second harmonicwill have a boost of about 4 time in amplitude, or 12 dB, due to thevariation of the real part of the specific acoustic impedance withfrequency. So the second harmonic pressure term in equation (8) isactually four times the power or 6 dB higher than the fundamental term.If the second harmonic falls above the cutoff frequency then the boostbegins to fall back to 0 dB. However, for most of the speech spectrumthere is a sever distortion and strong boost of the high frequencydistortion components.

Results for Two Tone AM Modulation Analysis

[0049] Because of the distortion attending single tone modulation,predistortion of the modulation could be attempted such that theresulting demodulated pressure wave will not contain harmonicdistortion. This will not work, however, because of the non-linearcross-products of two-tone modulation are quite different from singletone modulation as shown below.

[0050] Nevertheless, two-tone modulation distortion provides an insightfor the design of a corrective process for a complex modulation signalsuch as speech. The nature of the distortion is defined in terms ofrelative amplitudes and frequencies.

[0051] Equation (8b) is that of an AM modulated carrier for the two-tonecase where ω_(a1) and ω_(a2) are of equal amplitude and togethermodulate the carrier to a maximum peak value of 100 percent. The totalmodulated RF signal is given by equation (8b), below:

V(t)=A _(c) sin(ω_(c) t)[1+½ sin(ω_(a1) t)+{fraction (1/2)} sin(ω_(a2)t)]  (8b)

[0052] The square of (8b) is the power signal, which has the same formas the particle velocity, U_(r)(t), of equation (9), below.

[0053] From the square of (8b) the following frequencies and relativeamplitudes are obtained for the particle velocity wave, U_(r)(t), whichare in the audio range;

U _(r)(t)=C[sin(ω_(a1) t)+sin(ω_(a2) t)+¼ cos(ω_(a1)−ω_(a2))t)+¼cos(ω_(a1)+ω_(a2))t)−⅛ cos(2ω_(a1) t)−⅛ cos(2ω_(a2) t)]  (9)

[0054] If the frequencies in equation (9) are below the cut-offfrequency, the impedance boost correction will result in a pressure wavewith relative amplitudes given in equation (9a), below:

p(t)=C′[sin(ω_(a1) t)+b ² sin(ω_(a2) t)+(1−b ²)/4cos(ω_(a1)−ω_(a2))t)+(1+b ²)/4 cos(ω_(a1)+ω_(a2))t)−½ cos(2ω_(a1))t)−b²/2 cos(2ω_(a2) t)  (9a)

[0055] where: b=ω_(a2)/ω_(a1) and ω_(a2)>ω_(a1)

[0056] Equation (9a) contains a correction factor, b, for the specificacoustic impedance variation with frequency. The first two terms of (9a)are the two tones of the input modulation with the relative amplitudesmodified by the impedance correction factor. The other terms are thedistortion cross products which are quite different from the single tonedistortion case. In addition to the second harmonics, there are sum anddifference frequencies. From this two-tone analysis it is obvious thatmore complex multiple tone modulations, such as speech, will be severelydistorted with even more complicated cross-product and sum anddifference components. This is not unexpected since the process whichcreates the distortion is nonlinear. This leads to the conclusion that asimple passive predistortion filter will not work on a speech signalmodulated on an RF carrier by a convention AM process, because thedistortion is a function of the signal by a nonlinear process.

[0057] However, the serious distortion problem can be overcome by meansof the invention which exploits the characteristics of a different typeof RF modulation process in addition to special signal processing.

[0058] AM Modulation With Fully Suppressed Carrier for the IntelligibleEncoding of Speech by the Invention for Compatibility With the RFHearing Phenomena

[0059] The equation for AM modulation with a fully suppressed carrier isgiven by equation (10), below:

V(t)=a(t)sin(ω_(c) t)  (10)

[0060] This modulation is commonly accomplished in hardware by means ofa circuit known as a balanced modulator, as disclosed, for example in“Radio Engineering”, Frederick E. Termnan, p.481-3, McGraw-Hill, 1947.

[0061] The power signal has the same form as the particle velocitysignal which is obtained from the square of equation (10) as shown inequation (11), below:

P(t)=C U _(r) =a(t)²/2−(a(t)²/2)cos(2ω_(χ) t))  (11)

[0062] From inspection of equations (10) and (11) it is seen that, ifthe input audio signal, a(t), is pre-processed by taking the square rootand then modulating the carrier, the audio term in the particle velocityequation will be an exact, undistorted, replication of the input audiosignal. Since the audio signal from a microphone is bipolar, it must bemodified by adding a very low frequency (essential d.c.) bias term, A,such that the resultant sum, [a(t)+A]>0.0, is always positive. This isnecessary in order to insure a real square root. The use of a customdigital speech processor implements the addition of the term A, i.e. asshown in equation (10*), below:

V(t)=(a(t)+A)^(½) sin(ω_(c) t)  (10*)

[0063] The pressure wave is given by equation (11*), below:

p(t)=C U _(r) =A/2+a(t)/2−(a(t)/2)cos(2ω_(c) t)−(A/2)cos(2ω_(c)t)  (11*)

[0064] When the second term of the pressure wave of equation (11*) isprocessed through the specific acoustic impedance it will result in thereplication of the input audio signal but will be modified by the filtercharacteristics of the Real part of the specific acoustic impedance,R_(e){Z_(s)(f)}, as given in equation (8a). The first term of equation(11*) is the d.c. bias, which is added to obtain a real square root; itwill not be audible or cause distortion. The third and fourth terms of(11 *) are a.c. terms at twice the carrier frequency and therefore willnot distort or interfere with the audio range signal, a(t).

[0065] Since the filter characteristic of equation (7) is a linearprocess in amplitude, the audio input can be predistorted before themodulation is applied to the carrier and then the pressure or wound waveaudio signal, which is the result of the velocity wave times theimpedance function, R_(e){Z_(s)(f)}, will be the true replication of theoriginal input audio signal.

[0066] A diagram illustrating the overall system 30 and process of theinvention is shown in FIG. 3. Then input signal a(t) is applied to anAudio Predistortion Filter 31 with a filter function As(f) to produce asignal a(t)As(f), which is applied to a Square Root Processor 32,providing an output=(a(t)As(f)+A)^(½), which goes to a balancedmodulator 33. The modulation process known as suppressed carrier,produces a double sideband output=(a(t)As(f)+A)^(½) sin(ω_(c)t), whereω_(c) is the carrier frequency. If one of the sidebands and the carrierare suppressed (not shown) the result is single sideband (SSB)modulation and will function in the same manner discussed above for thepurposes of implementing the invention. However, the AM double sidebandsuppressed carrier as described is more easily implemented.

[0067] The output of the balanced modulator is applied to a sphericaldemodulator 34, which recovers the input signal a(t) that is applied tothe inner ear 35 and then to the acoustic receptors in the brain 36.

[0068] The various components 31-33 of FIG. 3 are easily implemented asshown, for example by the corresponding components 41-42 in FIG. 4,where the Filter 41 can take the form of a low pass filter, such as aconstant-K filter formed by series inductor L and a shunt capacitor C.Other low-pass filters are shown, for example, in the ITT FederalHandbook, 4th Ed., 1949. As a result the filter output is AS(f) a 1/f².The Root Processor 42 can be implemented by any square-law device, suchas the diode D biased by a battery B and in series with a largeimpedance (resistance) R, so that the voltage developed across the diodeD is proportional to the square root of the input voltage a(t)As(f). Thebalanced modulator 43, as discussed in Terman, op.cit., has symmetricaldiodes A1 and A2 with the modulating voltage M applied in opposite phaseto the diodes A1 and A2 through an input transformer T1, with thecarrier, O, applied commonly to the diodes in the same phase, while themodulating signal is applied to the diodes in opposite phase so that thecarrier cancels in the primary of the output transformer T2 and thesecondary output is the desired double side band output.

[0069] Finally the Spherical Demodulator 45 is the brain as discussedabove, or an equivalent mass that provides uniform expansion andcontraction due to thermal effects of RF energy.

[0070] The invention provides a new and useful encoding for speech on anRF carrier such that the speech will be intelligible to a human subjectby means of the RF hearing demodulation phenomena. Features of theinvention include the use of AM fully suppressed carrier modulation, thepreprocessing of an input speech signal be a compensation filter tode-emphasize the high frequency content by 40 dB per decade and thefurther processing of the audio signal by adding a bias terms to permitthe taking of the square root of the signal before the AM suppressedcarrier modulation process.

[0071] The invention may also be implemented using the same audio signalprocessing and Single Sideband (SSB) modulation in place of AMsuppressed carrier modulation. The same signal processing may also beused on Conventional AM modulation contains both sideband and thecarrier; however, there is a serious disadvantage. The carrier is alwayspresent with AM modulation, even when there is no signal. The carrierpower does not contain any information but contributes substantially tothe heating of the thermal-acoustic demodulator, i.e. the brain, whichis undesirable. The degree of this extraneous heating is more than twicethe heating caused by the signal or information power in the RF signal.Therefore conventional AM modulation is an inefficient and poor choicecompared to the double side-band suppressed carrier and the SSB types oftransmissions.

[0072] The invention further may be implemented using various degrees ofspeech compression commonly used with all types of AM modulation. Speechcompression is implemented by raising the level of the low amplitudeportions of the speech waveform and limiting or compressing the highpeak amplitudes of the speech waveform. Speech compression increases theaverage power content of the waveform and thus loudness. Speechcompression introduces some distortion, so that a balance must be madebetween the increase in distortion and the increase in loudness toobtain the optimum result.

[0073] Another implementation is by digital signal processing of theinput signal through to the modulation of the RF carrier.

What is claimed is:
 1. A method of producing undistorted subjectivesound, which comprises the steps of: pre-processor filtering amodulating signal; and modulating a fully suppressed carrier by thepreprocessor filtered modulating signal.
 2. The method of claim 1wherein said carrier is suppressed carrier amplitude modulated
 3. Themethod of claim 1 wherein said preprocessor filtering is of an audiospeech signal.
 4. The method of using the RF hearing phenomena,comprising the steps of: providing a model of a radio-frequency toacoustic transducer; analyzing the model to drive a new modulationprocess which will permit the RFD hearing effect to be used for thetransmission of intelligible speech.
 5. The method of claim 1 whereinthe preprocessing is of a speech input signal to de-emphasize the highfrequency content of said signal.
 6. The method of claim 5 wherein thepreprocessing takes place with a signal reduction of about 40 dB perdecade.
 7. The method of claim 1 wherein further processing of thesignal then takes place by adding a bias and then extracting a root ofthe waveform.
 8. The method of claim 1 wherein the further processing isby taking the square root of said waveform.
 9. The method of claim 7wherein the resultant signal is used to modulate an RF carrier in the AMfully suppressed carrier mode.
 10. The method of claim 9 wherein themodulated RF signal is demodulated by an RF to an acoustic process thatproduces an intelligible acoustic replication of the original inputspeech.
 11. The method of claim 10 wherein the demodulation is by athermal to acoustic process.
 12. The method of claim 10 wherein thedemodulation is by energy absorption which causes mechanical expansionin a medium and produces an acoustic signal.
 13. The method of claim 12wherein the demodulation is by energy absorption in an animal head tocause said mechanical expansion and said acoustic signal.
 14. The methodof claim 12 wherein the expansion in said head produces an acousticsignal which is passed by conduction to an inner ear where said signalis further processed in the same manner as an acoustic signal from anouter ear.
 15. A system for producing a modulated carrier from an inputmodulating signal, which comprises: a predistortion filter for saidinput signal; and means for modulating a fully suppressed carrier by thepreprocessor filtered modulating signal.
 16. The system of claim 15wherein said carrier is amplitude modulated.
 17. The system of claim 15wherein said predistortion filter de-emphasizes the high frequencycomponents of audio speech. 18 The system of claim 15 further includingan RF to acoustic transducer.
 19. The system of claim 17 wherein thepreprocessing takes place with a signal reduction of about 40 dB perdecade.
 20. The system of claim 17 wherein further processing of thesignal then takes place by adding a bias and then extracting a root ofthe waveform.